This invention relates to pressure-sensitive conductive elastomer compounds and, more specifically, to a pressure-sensitive conductive elastomer compound of the type which exhibits high resistance (insulating performance) when it is in non-pressed condition, and of which the resistance, as the compound is pressed, will vary according to the magnitude of the pressure.
Hitherto, pressure-conductive materials have been known which are in the form of a conductive compound comprising a resilient material, such as rubber or the like, and a conductive filler mixed therewith. For such filler, metallic particles, such as nickel, conductive carbon black, graphite particles and the like are normally used. Such conductive compound, molded into a rod or sheet form, is widely used today as a switching element, or as a pressure-sensitive element for sensors such as pressure sensor and tactile sensor.
Conductive compounds of such conventional type have the following difficulties. Those incorporating metallic particles as a conductive filler are liable to change of properties with time due to oxidation of the particles; therefore, they lack stability and are often subject to chattering and noise generation. Those incorporating powdery masses of a conductive carbon black as a conductive filler provide only insignificant change in resistance when they are under pressure, because the particle diameter of the carbon black is extremely small, i.e., 20.about.30 m.mu.; as such, they are of no practical use. If a granulated material formed of a conductive carbon black is used as such a filler, it is possible to provide greater variations in resistance, but a conductive compound incorporating such material is liable to particle breakage when it is under pressure; naturally, therefore, such compound lacks both durability and stability.
Where graphite particles are used as a conductive filler, no characteristic stability can be provided if they are of non-uniform shape as those of natural graphite. Therefore, it is known to use artificial graphite particles which have been rounded and freed of sharpness by pulverization, milling or otherwise to provide good characteristic stability.
Conductive compounds incorporating artificial graphite particles of such type are advantageous in that they are characteristically stable, durable, and less liable to noise generation, but on the other hand they have drawbacks in that preparation of graphite particles to the desired configuration requires a complicated and troublesome procedure and in that the attainable yield thereof is rather small.